The invention relates to a system for charging an energy store and to a method for operating the charging system of the invention.
The trend is that in the future electronic systems which combine new energy store technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power plants, and in vehicles, such as hybrid or electric vehicles. In conventional applications, an electrical machine which is in the form of a polyphase machine, for example, is controlled via a converter in the form of an inverter. A characteristic feature of such systems is a so-called DC voltage intermediate circuit, via which an energy store, generally a battery, is connected to the DC voltage side of the inverter. In order to be able to meet the requirements for a respective application placed on power and energy, a plurality of battery cells are connected in series. Since the current provided by such an energy store needs to flow through all of the battery cells and a battery cell can only conduct a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current.
A series circuit comprising a plurality of battery cells entails the problem, in addition to a high total voltage, that the entire energy store fails when a single battery cell fails because no battery current can flow any more. Such a failure of the energy store can result in failure of the entire system. In the case of a vehicle, a failure of the drive battery can render the vehicle “unusable”. In other applications such as the rotor blade adjustment of wind power plants, for example, hazardous situations may even arise in the event of unfavorable boundary conditions, such as a strong wind, for example. Therefore, a high degree of reliability of the energy store is always desired, whereby “reliability” is intended to mean the capacity of a system to operate fault-free for a predetermined time.
In the earlier applications DE 10 2010 027857 and DE 10 2010 027861, batteries having a plurality of battery module strings have been described which can be connected directly to an electrical machine. In this case, the battery module strings have a plurality of series-connected battery modules, wherein each battery module has at least one battery cell and an associated controllable coupling unit, which makes it possible, depending on control signals, to interrupt the respective battery module string or to bypass the respectively associated at least one battery cell or to connect the respectively associated at least one battery cell into the respective battery module string. By suitably actuating the coupling units, for example with the aid of pulse-width modulation, it is also possible for suitable phase signals for controlling the electrical machine to be provided, with the result that a separate pulse-controlled inverter is not required. The pulse-controlled inverter required for controlling the electrical machine is therefore integrated in the battery, so to speak. For the purposes of the disclosure, these two earlier applications are incorporated in full in the present application.